The present invention relates to a sewing machine, and more particularly to a needle oscillating mechanism thereof equipped with a cam member having a cam surface to control the position of lateral oscillation of a needle, a step motor for rotating the cam member in both forward and backward directions, drive control means for driving the step motor synchronously with rotation of a main shaft, and a contact finger engaged with the cam surface of the cam member and serving to determine the position of lateral oscillation of the needle in accordance with the cam surface.
There are known sewing machines of the above structure wherein the cam member is rotated synchronously with the main shaft of the machine by means of the step motor and the position of lateral oscillation of the needle is controlled by the shift of the contact finger engaged with the cam surface of the cam member. For the purpose of reducing a load applied to the step motor during its rotation in such a sewing machine, one prior art is executed by first disengaging the contact finger from the cam member synchronously with rotation of the main shaft, then rotating the cam member by means of the step motor, and subsequently bringing the contact finger into engagement with the cam member again.
The sewing machine mentioned above incorporates many moving elements including a needle bar to which a needle is secured, a needle bar gate swingably mounted on the machine frame and supporting the needle bar for endwise reciprocation and lateral oscillation thereof, and members for operatively connecting the needle bar gate and the contact finger. And such elements are once moved to the respective extreme positions of movement thereof in response to each disengagement of the contact finger from the cam member, and then are moved finally to the positions corresponding to engagement of the contact finger with the cam member. Accordingly, even in the case of straight stitching where the needle motion required is merely continuous fall onto the same lateral position, there occurs unnecessary idle motion of the needle in the lateral direction thereof. Such a phenomenon confuses an operator or gives her an unpleasant feeling, and furthermore, due to a large total mass of the moving elements, a great force of inertia is exerted on them during a high-speed operation mode of the sewing machine, thereby generating loud impact sound at the time of engagement of the contact finger with the cam member, and impeding quick determination of the needle position for each needle penetration through the work fabric. These cause a disordered stitch pattern as a result.